The present invention relates to the production of calcium fluoride and more especially, to the preparation of calcium fluoride from hexafluoro silicic acid.
In the past several years several processes have become known which make possible the preparation of hydrogen fluoride, the most important starting material compound in fluorine chemistry, from fluorosilicic acid. As far is known, however, none of these processes is being commercially practiced. The reason for this is to be seen in the high process costs.
For example, there are already known processes according to which calcium fluoride can be obtained from fluorosilicic acid by reaction with calcium carbonate CaCO.sub.3 or calcium hydroxide Ca(OH).sub.2. In this regard, U.S. Pat. No. 2,780,521 and No. 2,780,523 describe the reaction of diluted fluorosilicic acid having a content of between 2.5 and 3.8% of H.sub.2 SiF.sub.6 with lime, according to the reaction equation EQU H.sub.2 SiF.sub.6 + 3 CaCO.sub.3 .fwdarw. 3 CaF.sub.2 + SiO.sub.2 .multidot. H.sub.2 O + 3 CO.sub.2
if the reaction is carried out in a pH range of 3.5-6.7 (according to claim 2 preferably in the range 5.5-6.5), calcium fluoride CaF.sub.2 is said to be separable in an easily filterable form from the concurrently produced SiO.sub.2 -sol.
A considerable disadvantage of the process in accordance with the first-mentioned patent resides in the fact that there is obtained a calcium fluoride product which contains between 4 and 7% SiO.sub.2 as an impurity. Such a material has little suitability for the production of hydrogen fluoride.
In accordance with the second patent, there is obtained a calcium fluoride product having only a small content of SiO.sub.2 (about 0.5% and less). This is said to be accomplished by providing that not more than 85% of the amount of calcium carbonate CaCO.sub.3 stoichiometrically required for the formation of calcium fluoride is added to the diluted silicic acid. After filtration of the product obtained in this manner, the filtrate is once more treated with calcium carbonate until a pH value of between 7 and 7.3 has been achieved. The product yielded in this step constitutes a mixture of calcium fluoride and calcium silicofluoride which is not useable. In addition to this disadvantage of the process, a further disadvantage is to be seen in the fact that relatively long action periods of approximately 30 minutes to two hours are required in connection with the process.
An especially considerable weakness of both processes set forth in the foregoing United States patents resides, however, in the fact that only relatively strongly diluted fluorosilicic acid, maximum concentration 4-6%, may be employed. The reason for this may be traced back to the known phenomenon that silica salts are unstable and depending upon parameters such as pH value, temperature, time, and foreign ion content tend toward separation of SiO.sub.2 with concurrent gel formation.
Accordingly, the industrial production of hydrogen fluoride still proceeds today exclusively from natural fluor spar which is reacted in the form of the so-called acid grade spar with more than 97% calcium fluoride CaF.sub.2.
There exists, therefore, a definite need to convert into calcium fluoride the fluoro silicic acid produced as a by-product in ever increasing amounts during the production of super phosphates and wet phosphoric acid. In this manner, the fluoride obtained in such a form can be immediately converted to hydrogen fluoride in existing industrial installations. Moreover, calcium fluoride represents an unproblematic fluorine reserve, since it may be safely deposited in piles.